pull in buildImage package from nixpkgs for docker module evaluation

[hall]

In order to evaluate the docker module (for generating documentation), I had to expand the default value (in e3127e8) of image.tag and image.name to include an empty string if config.image is null. It's likely that pkgs.dockerTools.buildImage (where the imageName and imageTag attrs are built) needs to be referenced in some way to avoid config.image being null.

I'm not confident at this time whether that's actually true but I'm creating this issue to not lose track of the (potential) problem.